It is well known that a metal or plastic tube can be extruded in a straight length and subsequently coiled into a helix. However, when a tube is thus bend or coiled, the developed length of the radially outer portion of the tube wall will of necessity be greater than the length of the radially inner portion. Unless special precautions are taken, the conflict between these dimensions is at least partially solved by collapsing of the tube.
As an example, if a 1" outer diameter tube (d=1") is bent through 360.degree. with a 12" inner radius (R=12"), the developed length of the inside wall of the tube will be 75.398" (2.pi.R), and that of the outer wall will be 81.681" (2.pi.(R+d)), a difference of 6.283". In a ductile metal tube this can to some degree be accommodated by stretching the outer wall and perhaps compressing the inner wall. In order to support the tube bore against collapse during the forming and stretching process, a coiled pipe-bending spring may be inserted into the tube bore, or the bore may be filled with a low melting, solder-like alloy. Furthermore, the inner form against which the tube is pressed and bent may be grooved to one-half the diameter of the tube in order to give passive support to the tube.
The above-mentioned methods do not work well with plastic tubes, because such stretching of the outer radius as may be achieved is usually within the elastic limits of the material. Consequently, when the tube is released from the bending device, it immediately springs back to its original straightness, although some slight curvature may be retained.
Attention is directed to the following U.S. patents:
Bennett, et al. T873,007; Apr. 14, 1970 PA0 Fraser 1,584,283; May 11, 1926 PA0 Campbell 2,039,475; May 5, 1936 PA0 Moustier 3,296,661; Jan. 10, 1967 PA0 Kuehnle 3,207,827; Sep. 21, 1965 PA0 Harp, Jr. 3,274,313; Sep. 20, 1966 PA0 Holmgren 3,454,695; Jul. 8, 1969 PA0 Bernett 3,825,641; Jul. 23, 1974 PA0 Goldsworthy, et al. 3,873,399; Mar. 25, 1975 PA0 Casaert, et al. 4,182,738; Jan. 8, 1980 PA0 Levens 4,273,523; Jun. 16, 1981 PA0 Levens 4,359,446; Nov. 16, 1982
Attention is also directed to the following foreign patents: Great Britain Patent No. 17,797; Japanese Patent No. 54-52174; Japanese Patent No. 59-179320; and Japanese Patent No. 42-4076.